There are numerous other types of processes for remediating soils, including soil washing, in-situ air stripping, in-situ vitrification, stabilisation, vacuum extraction and solvent extraction. However, the most universally proven and efficient method for removing organics from soil is thermal desorption, which together with treatment or destruction of the desorbed organics is termed thermal remediation. Hydrocarbon contaminants which are treatable with thermal remediation include:                Volatile organic compounds (VOC) eg petrol, diesel,        Aromatic hydrocarbons eg benzene, tars,        Dioxins and furans,        Semi-volatile organic compounds (SVOCs),        Polynuclear aromatic hydrocarbons (PAHs or PNAs),        Polychlorinated biphenyls (PCBs), and        Pesticides (eg organochlorins such as dieldrin and aldrin).        
Thermal remediation of contaminated soil uses heat to physically separate hydrocarbon based contaminants from feed material which may be, for example, directly recovered soils, sediments, sludges or filter cakes. The separated hydrocarbons are then combusted or thermally oxidised to produce essentially carbon dioxide and water vapour.
The most common process configuration involves a counter-current direct fired desorber, but there are numerous variants. The most common alternative is the co-current desorber, which produces a hotter contaminated offgas stream. To avoid cooling these gases to enable fabric filtration, a cyclone is used to remove some of the dust prior to thermal oxidation, followed by gas cooling then fabric filtration. In another variant the functions of the thermal desorber and oxidiser are combined by arranging to combust the contaminant gases within a metal jacketed combustion chamber within a rotary desorber.
U.S. Pat. No. 5,658,094 discloses an arrangement in which heat exchangers are used for preheating combustion air for a thermal desorber. In that arrangement, there is described a combined (all metal) rotary device, a type of rotary kiln with internal indirect heating of both soil and combustion air, which is claimed to carry out combined thermal desorption and thermal oxidation.
German patent application 3447079 describes a process in which the contaminated soil is thermally treated in a rotary kiln by the direct addition of hot combustion gases and/or air. The decomposition products are partially combusted in the rotary kiln, with the remaining production gas fed to a waste gas combustion chamber where it is afterburnt at high temperatures. In general, the post-combustion waste gases are cooled and released into the atmosphere.
Various other methods of thermal remediation of soil are described in U.S. Pat. Nos. 5,455,005, 5,393,501, 4,715,965, 4,974,528, and 5,378,083.
The main difference between different technologies is the equipment used for thermal desorption, which may be one of four main types, the advantages and disadvantages of which are summarised in Table 1 (obtained from various sources, including W. L. Troxler et al, “Treatment of non-hazardous petroleum-contaminated soils by thermal desorption technologies”, Jnl of Air and Waste, Vol. 43, November 1993, and W. C. Anderson, “Innovative site remediation technology”, Thermal Desorption, WASTECH, 1993).
TABLE 1Main Types of Thermal RemediationAdvantagesDisadvantagesDirectHigh rates ofLarger thermal oxidiser than forfiredheat transfer.indirect fired.rotarySmaller desorber thanDilution strategies are usuallykilnsindirect fired.required for hydrocarbonSimplest, most robust.contamination levels of >4% toMost flexible toavoid exceeding the LEL ofvariation in feeddesorber offgases.material and typeand level ofcontamination.IndirectMay allow economicUnsuitable for heavyfiredrecovery ofcontamination, especially of longrotaryhydrocarbons.chain or aromatic hydrocarbonskilnsLower dust losses(tars).from desorber.Larger desorber.Higher moisture soils severelyimpair capacity.CombinationProcess simplificationInability to process large gasdirect/by using a singlevolumes.indirectprocess step.Lower peak soil temperatures willfiredprevent practical decontaminationdesorber,of heavily contaminated soils,withespecially with PAHs or PCBs.integralLess suitable for high moisturethermalsoils.oxidiserDirectAs for indirectAs for indirect fired rotary kilns.firedfired rotary kilns.conveyors,Improved control overincludingsolids residence time.metalbelts andscrewsDirectHighest processIncreased complexity.firedintensity.Increased dust losses/recyclingfluidisedof dusts.bedsRequires fine and uniform sizedmaterial (normally lessthan 5 mm).Increased maintenance (abrasion).
Remediation plants may be either stationary or mobile, with the larger, stationary plants being restricted to remediation of large heavily contaminated sites (eg large integrated steelworks sites), regional clusters of contaminated sites, or under circumstances where transport of contaminated materials is economic and not hazardous.
Key technical factors in thermal remediation include:                Solids temperature and contact time.        Soil moisture when treated.        Actual soil hydrocarbon contaminants present.        Other contamination, eg chlorine compounds and heavy metals.        Extraneous rubble.        
It is an object of the present invention to provide an improved method and apparatus for remediating soil contaminated with hydrocarbons that is capable in preferred embodiment of optimising energy usage and operating costs for a given soil throughput, and that is preferably adaptable to treat short chain, long chain, aromatic, and polychlorinated hydrocarbons. In particular embodiments, it is further desired to minimise environmental impacts, especially greenhouse gases, NOx and dioxin/furan emissions.